This invention relates to an ignition circuit for spark plugs, and more specifically relates to an IGBT for controlling the breakdown of a spark plug and a novel clamping circuit which reduces the necessary die area for the IGBT.
Ignition circuits for spark plugs are well known, wherein a switching device turns on to excite the primary winding of an ignition transformer from a d-c source. When the switch turns off, a sufficiently high voltage is induced in the secondary winding which is connected across the spark gap to cause a spark between the spark gap electrodes.
The switching device may take many forms, for example, mechanical contacts and semiconductor switching devices. MOSgated power semiconductor devices have been used, for example, power MOSFETs, IGBTs.
IGBTs used in spark plug ignition systems have been required by customer specifications to satisfy two conditions. The first is to have a maximum forward voltage drop V.sub.CEON of 1.8 volts or less at 7 amperes and at 125.degree. C. This condition can be satisfied by an IGBT having a die size of 14 mm.sup.2 (size 2), which is a relatively inexpensive die.
However, the second condition requires a die size of 25 mm.sup.2 (size 4) which is more expensive. The second condition is the repetitive unclamped energy specification W.sub.CER =200 mJ for a collector current of 12 amperes and a starting die temperature Tj of 100.degree. C. This condition results from the fact that all the energy stored in the coil is dissipated in the IGBT if the spark does not occur (as due to a disconnected spark plug).
It would be very desirable to reduce the power dissipation in the power switching device under such conditions so that a smaller die size device can be used for spark plug ignition circuit application.